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    <title>Document</title>
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    <script>
        //XHR对象
        const getJSON = function(){
            const promise = new Promise(function(resolve,reject){
                //异步操作：网络请求代码
            const handler = function(){
                if(this.readyState !== 4){
                   //0 1 2 3 4 
                   return;
                }
                if(this.status === 200){
                    resolve(this.response);
                }else{
                    reject(new Error(this.statusText))
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            client.open("GET",url);
            client.onreadystatechange = handler;
            client.responseType = "json";
            client.setRequestHeader("Accept","application/json");
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            return promise;
        }

        getJSON("")
        .then(function(data){
            console.log(data);
        },function(error){
            console.log(error);
        })

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